I've been reviewing a TomTom Go930 for the past weeks; and it is pretty close to what I'd call perfect. The mapshare feature is great, and allows you to make your own map corrections (and submit them to tomtom).

If you find map errors on Garmin or Magellan systems, go to the Navteq mapreporter site and report them: http://mapreporter.navteq.com/

They supply these maps, and any corrections need to be made at the source.